Wireless telecommunication systems are well known in the art. In order to provide global connectivity for wireless systems, standards have been developed and are being implemented. One current standard in widespread use is known as Global System for Mobile Telecommunications (GSM). This is considered as a so-called Second Generation mobile radio system standard (2G) and was followed by its revision (2.5G). GPRS and EDGE are examples of 2.5G technologies that offer relatively high speed data service on top of (2G) GSM networks. Each one of these standards sought to improve upon the prior standard with additional features and enhancements. In January 1998, the European Telecommunications Standard Institute—Special Mobile Group (ETSI SMG) agreed on a radio access scheme for Third Generation Radio Systems called Universal Mobile Telecommunications Systems (UMTS). To further implement the UMTS standard, the Third Generation Partnership Project (3GPP) was formed in December 1998. 3GPP continues to work on a common third generational mobile radio standard.
A typical UMTS system architecture in accordance with current 3GPP specifications is depicted in FIG. 1. The UMTS network architecture includes a Core Network (CN) interconnected with a UMTS Terrestrial Radio Access Network (UTRAN) via an interface known as Iu which is defined in detail in the current publicly available 3GPP specification documents. The UTRAN is configured to provide wireless telecommunication services to users through wireless transmit receive units (WTRUs), known as User Equipments (UEs) in 3GPP, via a radio interface known as Uu. The UTRAN has one or more Radio Network Controllers (RNCs) and base stations, known as Node Bs in 3GPP, which collectively provide for the geographic coverage for wireless communications with UEs. One or more Node Bs are connected to each RNC via an interface known as Iub in 3GPP. The UTRAN may have several groups of Node Bs connected to different RNCs; two are shown in the example depicted in FIG. 1. Where more than one RNC is provided in a UTRAN, inter-RNC communication is performed via an Iur interface.
Communications external to the network components are performed by the Node Bs on a user level via the Uu interface and the CN on a network level via various CN connections to external systems.
In general, the primary function of base stations, such as Node Bs, is to provide a radio connection between the base stations' network and the WTRUs. Typically a base station emits common channel signals allowing non-connected WTRUs to become synchronized with the base station's timing. In 3GPP, a Node B performs the physical radio connection with the UEs. The Node B receives signals over the Iub interface from the RNC that control the radio signals transmitted by the Node B over the Uu interface.
A CN is responsible for routing information to its correct destination. For example, the CN may route voice traffic from a UE that is received by the UMTS via one of the Node Bs to a public switched telephone network (PSTN) or packet data destined for the Internet. In 3GPP, the CN has six major components: 1) a serving General Packet Radio Service (GPRS) support node; 2) a gateway GPRS support node; 3) a border gateway; 4) a visitor location register; 5) a mobile services switching center; and 6) a gateway mobile services switching center. The serving GPRS support node provides access to packet switched domains, such as the Internet. The gateway GPRS support node is a gateway node for connections to other networks. All data traffic going to other operator's networks or the internet goes through the gateway GPRS support node. The border gateway acts as a firewall to prevent attacks by intruders outside the network on subscribers within the network realm. The visitor location register is a current serving network's ‘copy’ of subscriber data needed to provide services. This information initially comes from a database which administers mobile subscribers. The mobile services switching center is in charge of ‘circuit switched’ connections from UMTS terminals to the network. The gateway mobile services switching center implements routing functions required based on current location of subscribers. The gateway mobile services also receives and administers connection requests from subscribers from external networks.
The RNCs generally control internal functions of the UTRAN. The RNCs also provide intermediary services for communications having a local component via a Uu interface connection with a Node B and an external service component via a connection between the CN and an external system, for example overseas calls made from a cell phone in a domestic UMTS.
Typically an RNC oversees multiple base stations, manages radio resources within the geographic area of wireless radio service coverage serviced by the Node Bs and controls the physical radio resources for the Uu interface. In 3GPP, the Iu interface of an RNC provides two connections to the CN: one to a packet switched domain and the other to a circuit switched domain. Other important functions of the RNCs include confidentiality and integrity protection. Background specification data for such systems are publicly available and continue to be developed.
In general, commercial wireless systems utilize a well defined system time frame format for the transmission of wireless communication signals. In communication systems such as Third Generation Partnership Project (3GPP) Time Division Duplex (TDD) and Frequency Division Duplex (FDD) systems, multiple shared and dedicated channels of variable rate data are combined for transmission. However, irrespective of whether a system is based on TDD or FDD, received wireless signals must be decoded in accordance with the timeframe structure with which they are transmitted.
One of the first tasks to be performed in the initiation of a wireless communication is to determine the relative timing of a received signal for synchronization. In modern systems, there are various levels of synchronization, such as, carrier, frequency, code, symbol, frame and network synchronization. At each level, synchronization can be divided into two phases: acquisition (initial synchronization) and tracking (fine synchronization).
A typical wireless communication system, such as specified in the 3rd Generation Partnership Project (3GPP), sends downlink communications from a base station to one or a plurality of User Equipments (UEs) and uplink communications from UEs to the base station. A receiver within each UE operates by correlating, or despreading, a received downlink signal with a known code sequence. The code sequence is synchronized to the received sequence in order to get the maximal output from the correlator.
A receiver may receive time offset copies of a transmitted communication signal known as multi-path. In multi-path fading channels, the signal energy is dispersed over a certain amount of time due to distinct echo paths and scattering. To improve performance, the receiver can estimate the channel by combining the multi-path copies of the signal. If the receiver has information about the channel profile, one way of gathering signal energy is then to assign several correlator branches to different echo paths and combine their outputs constructively. This is conventionally done using a structure known as a RAKE receiver.
Conventionally, a RAKE receiver has several “fingers”, one for each echo path. In each finger, a path delay with respect to some reference delay, such as the direct or the earliest received path, must be estimated and tracked throughout the transmission. The estimation of the path's initial position in time may be obtained by using a multi-path search algorithm. The multi-path search algorithm does an extensive search through correlators to locate paths with a desired chip accuracy. RAKE receivers are able to exploit multi-path propagation to benefit from path diversity of transmitted signal. Using more than one path, or ray, increases the signal power available to the receiver. Additionally, it provides protection against fading since several paths are unlikely to be subject to a deep fade simultaneously. With suitable combining, this can improve the received signal-to-noise ratio, reduce fading and ease power control problems.
During reception, it is not always possible to separate the received energy into components attributable to distinct multipath components. This may happen, for example, if the relative delays of the various arriving paths are very small compared to the duration of a chip. Such situations often arise in indoor and urban communication channels. The problem is often referred to as the “Fat Finger Effect.” Accordingly, RAKE receivers have been developed that are capable of identifying the Fat fingers, such as the RAKE receiver disclosed in U.S. patent application Ser. No. 10/304,894, RECEIVER FOR WIRELESS TELECOMMUNICATION STATIONS AND METHOD published as Publication No. US-2003-0157892-A1 on Aug. 21, 2003 and owned by the assignee of the present invention. FIG. 2 is an illustration of the processing of a received wireless communication signal with a preferred RAKE receiver that includes Fat finger allocation.
As illustrated in FIG. 2, the received wireless communication system is subject to an initial cell search preprocessing before RAKE finger allocation. The initial preprocessing identifies reception of a specific signal sequence such as a pilot sequence or, for example, a preamble sequence of a Random Access Channel (RACH). Various methods of searching for and identifying known transmitted signal sequences are know in the art. For example, such methods and apparatus are disclosed in U.S. patent application Ser. No. 10/322,184, APPARATUS AND METHOD OF SEARCHING FOR KNOWN SEQUENCES published as Publication No. US-2003-0161416 on Aug. 28, 2003 and owned by the assignee of the present invention.
There are several purposes why a sequence of symbols known to the receiver might be sent out from a transmitter, for example, channel estimation with respect to timing delay, amplitude and phase such as in a path search; signaling for (slotted) ALOHA multiple access collision detection and access granting such as with RACH preamble detection; and signaling of timing relations and even code group allocations, such as in a cell search.
In cases where lower level signaling is involved, there are usually several different known sequences that possibly can be sent out, and the signaling value is dependent on which one is found. Therefore, the search has to be performed over all available possible, or relevant, sequences.
The exact receive timing of a known sequence is often not known. Unfortunately, this is exactly the parameter of interest, e.g., for RACH preamble, if the distance and therefore the propagation latency between transmitter and receiver are not known. Additionally, the transmit timing can be completely unknown, such as in cell searching; or the reception of the known sequence could be in different replicas with respect to timing, amplitude and phase, but these parameters would then be of particular interest, such as in path searching.
In general, there is a certain time window when the sequence is expected to be received, which is constituted by some transmit timing relationship, or simply the repetition rate if the sequence is repeatedly sent out on a regular basis. Therefore, on the receive side, a search for the sequence is made within the time window, typically by repeated correlation of the incoming received signal at consecutive instances in time followed by a search of maxima or threshold comparison in the output signal of the correlator. This operation of correlation at consecutive time instances can be viewed as finite impulse response (FIR) filtering of the incoming signal using the expected sequence as the coefficients for the FIR filter. This is in line with the idea of using a matched filter for detection.
In a 3GPP system, the known sequences of symbols are transmitted using a pulse shaping filter of the root-raised-cosine (RRC) type. On the receiver side, an RRC-type filter matched to this transmit pulse is used. The combination of both filters, in time domain the convolution, is then of the raised-cosine (RC) type. FIG. 3 shows an impulse response of an RC filter in time domain, with a filter roll-off factor of 0.22 as used in 3GPP, and being normalized to 1.0 as the maximum amplitude. Amplitude magnitude in dB of the impulse response for the filter of FIG. 3, is shown in FIG. 4.
If the transmit and receive timing for a symbol are fully aligned, the received signal amplitude is at maximum and for neighboring symbols spaced at integer multiples of the symbol duration Tc, the received signal is zero. This is one of the essential properties of these types of filters and is the reason why this type of filter is used in this application.
If the exact symbol timing is not known, and the reception is off by some timing offset, then the received signal amplitude is not at maximum any more. With the search of a known sequence with unknown timing, the exact symbol timing will typically not be met. Accordingly, this type of error almost always occurs.
If the search for a known sequence is performed spaced in time at Tc, then the maximum possible timing error is Tc/2, and the amplitude degradation resulting from this, as shown in FIG. 4, is about 4 dB, which is prohibitive for performance reasons. For a sequence search performed spaced at Tc/2, the maximum timing error is Tc/4, and the amplitude degradation 0.94 dB.
In view of the above, performing full correlations at a rate of Tc/2 is the approach most widely seen in current approaches to the challenge of a known sequence search with unknown timing. For example, FIG. 5 shows a system model 10 in which a dirac pulse 12 is applied to a sequence FIR filter 14 which is applied to a root-raised cosine (RRC) FIR filter 18 forming part of the channel 16. At the receiver end, a root-raised-cosine (RRC) FIR filter 20 receives the transmitted signal, filter 20 being matched to the transmit pulse. The combination of the filters 18 and 20, function as a raised-cosine (RC) type filter. A known sequence detector 22 is used in the signal processing chain. After the interpolation, the post-processing, e.g., maximum search or threshold detection is performed at stage 22.
Omission of an FIR filter structure from the signal processing chain results in a search for the known sequence by correlation to either suffer from severe performance degradation or to require the already major chip rate processing complexity to be doubled. For example, FIG. 6 shows the “brute force” method wherein the known sequence detector 22 includes a correlator finite impulse response (FIR) filter 24, which receives the incoming signal at the rate of two samples per chip and provides its output to the peak search detector 25, likewise operating at the rate of two samples per chip.
By comparison, the implementation disclosed in Publication No. US-2003-0161416, referenced above and shown in FIG. 7, provides the incoming signal to the sequence correlator FIR filter 24 at the rate of one sample per chip. Its output, also at one sample per chip, is directly applied to a multiplexer 28 as well as an estimation filter 26, which preferably is a four (4)-tap FIR filter. The signal is applied to FIR filter 24 at the rate of one sample per chip and its output, likewise, at the one sample per chip rate, is processed by the estimation FIR filter 26. The multiplexer 28 receives the two signal streams and alternates passage of these streams to the peak search/detector 25 which performs the peak search/detection operation at a rate of two samples per chip. However, even this improved approach is not optimum with respect to the processing effort.